Field of the Invention
The present disclosure is generally related to the field of medical imaging, and more particularly the field of radiography. Embodiments of the present invention relate to the field of the deploying anti-scattering grids used to improve radiographic frames by filtering the photons scattered by the organ under study, and keeping only the photons emitted by the source. Embodiments of the present invention can be utilised within the scope of mammography, and more particularly within the scope of mammary tomosynthesis, which takes a series of frames at different angles to produce a 3D image of the object being studied.
Description of the Prior Art
Anti-scattering grids are used widely in radiography devices to eliminate effects due to parasite scattering of some photons, taking place in the organ studied in said devices. These grids filter photons scattered by the organ being studied and mainly keep only photons actually originating from the radiation source of the radiography device, thus improving the contrast on the images obtained.
In reference to FIG. 1, a conventional use of an anti-scattering grid 2 is illustrated. This grid 2 is placed between an organ of a patient P to be studied and who is irradiated by a radiation source 1, and a radiation detector 3 comprising a network of sensors 31 (illustrated in FIG. 3) distributed periodically with a period pd (later called the pitch of the detector).
The assembly made up of the grid and the detector is positioned according to a plane perpendicular to the plane PT of the torso of the patient. By way of non-limiting example, illustrated in FIG. 1, the patient P can be standing and the grid and the detector are then on a horizontal plane, the walls of the grid and of the detector in contact with the patient being tangential to the plane PT,
The grid 2 is generally constituted by alternating radio-opaque and radio-transparent laminates 21, the laminates 21 being parallel and distributed periodically, with a pitch pg between two radio-opaque laminates so that the scattered photons are absorbed by the radio-opaque laminates and the photons coming directly from the source 1 are transmitted to the detector 3 of the radiography device.
One drawback of using such a grid is that an image of the grid appears on the detector. Also, alternating laminates can cause interference figures, or moiré effect, on the detector and deteriorate the quality and legibility of the frame obtained.
For deleting the image of the grid, a solution known in mammography and schematically illustrated in FIGS. 1 and 2a is used, consisting of animating the grid 2 by vibration movement perpendicular to the direction according to which the laminates 21 extend, that is, parallel to the side of the grid against which the patient can be positioned.
In terms of mammary tomosynthesis, acquiring a 3D image of the object means acquiring a series of images of the object according to different relative angular positions between the source and the detector. For this to occur, the radiation source 1 is pivoted about an axis Y-Y, illustrated in FIG. 2b, perpendicular to the plane of the torso of the patient P, since pivoting of the source according to an axis parallel to the plane of the torso PT of the patient P would create risk of irradiating the latter.
This configuration necessarily causes the grid to pivot by 90° in its plane so that the trajectory of the source 1 remains near the focal line of the grid 2.
For reasons associated with bulk, as illustrated in FIG. 3, it then becomes difficult to execute displacement of the grid 2 perpendicularly to the direction of the laminates.
In effect, in reference to FIG. 3, the detector 3 and the anti-scattering grid 2 are located under a cap 4 likewise supporting the breast of the patient throughout examination. Legislation imposes that the distance between the costal grid of the patient and the closest sensors 31 of the detector 3 be less than 5 mm.
This interval must also comprise the thickness of the cap 4 and the edge of the grid 2 which is not constituted by laminates. Given these elements, the space remaining for the grid 2 to move is less than 2 mm.
Since the movement required to erase the image of the grid is of the order of 10 mm, it cannot occur according to an axis perpendicular to the plane of the torso PT of the patient.
There is therefore a need for a novel technique for making an image with a grid having laminates parallel to the chest of the patient P, while at the same time avoiding the image of the grid and the moiré effects on the detector.
Solutions adopted in mammary tomosynthesis in the prior art to eliminate the image of the grid on the detector propose adapting the pitch of the grid pg to the pitch of the detector pd, so that the pitch of the grid pg is for example equal to a multiple of the pitch pd of the detector.
Another solution presented in document FR 2,939,019 consists of adapting the pitch of the grid to the Nyquist frequency of the detector then digitally filtering the image of the grid on the detector.
But, none of these solutions gives a completely satisfactory result. In particular, even if the grid is no longer visible on the frame, a moiré effect remains, linked to interferences between the laminates of the grid and the network of sensors of the detector.